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M. BUR[ÁK et al.: INFLUENCE OF THE STRAIN RATE ON THE MECHANICAL AND TECHNOLOGICAL PROPERTIES...
For static tensile tests a universal test machine
INSTRON 1185 was used and this machine was used retooled
with an Erichsen test fixture for the deep-drawing
tests in the press tool velocity interval from 3,3·10 –3
m·s –1 to 1,10 –1 m·s –1 . Deep-drawing tests with speed
punch up to 2,5 m·s –1 were done by drop tower.
EXPERIMENTAL RESULTS AND DISCUSSION
The influence of strain rate on the basic mechanical
properties of tested steel C4 is shown in Figure 1, which
indicates that increased strain rates result in increased
strength properties, whereas the intensity of growth of Re
is higher than of Rm. The dependence of the strength properties
on the strain rate for un-alloyed high-grade steels in
the range from 10 –3 to 10 3 s –1 was described using parametric
equations, mostly in the form presented in 4,6,8,
Re Re k ln( / )
0
0
Rm Rm k ln( / )
0
0
where Re, Rm are the yield strength and the ultimate tensile
strength at the given strain rate , R and R , are
e0 m0
the yield strength and the tensile strength at the static
strain rate (10 –3 s –1 ).
When the material is more homogeneous and there is
the lower amount of obstacles for dislocations movement,
the more sensitive to the strain rate is. Figure 2
shows the influence of the strain rate on the increment of
the yield strength Re of C33 steel after various heat
treatments.
The as-quenched steel has the lowest sensitivity to ,
because the martensitic makrostructure has the highest
number of obstacles to dislocations movement, and the
as-normalized steel has the highest sensitivity.
Similarly, Figure 3 documents the influence of on
Reand Rmvaluesof C4 and E500TS steels with different
grain size.
The grain boundaries are insuperable obstacles to
dislocation movement, therefore the finer grain caused
the more obstructions, and the steel is less sensitive to
the strain rate . In terms of assessment of formability,
the Re/Rm ratio is the most important criterion.
Test results realized on different steel grades confirmed
fact that by increasing of strain rate the ratio
Figure 1 Influence of the strain rate on mechanical properties
of steel C4.
Figure 2 Influence of the strain rate on the increment of
strength properties Re or Rm, after heat treatment,
compared with the initial state ( =10 –3
s –1 ) of steel C33, after various heat treatments.
Figure 3 Influence of the strain rate on the increment of
strength properties ÄRe or ÄRm, compared with
the initial state at =10 -3 s -1 for various steel
grades.
Re/Rm is also increased and the intensity of that increase
is a function of material. The lower amount of obstructions
for dislocations movement in material causes the
more remarkable influence of . Figure 4 shows an example
of influence of grain size to ratio Re/Rm for different
strain rates.
Analysis of achieved results of strength properties
(Re, Rm) showed that un-alloyed high-grade steels dedicated
for cold forming up to critical strain rate kr allocated
the ratio Re/Rm < 1 and from the view of macro volume
up to that strain rate should be keeping the plastic
stability till deformation responded to stress Rm.
Figure 4 Influence of the strain rate on Re/Rm for various
steel grades.
318 METALURGIJA 49 (2010) 4, 317-320